1. Field of the Invention
The invention relates generally to seismic sensors, and more specifically to seismic sensors using leaf-springs to suspend the seismic mass.
2. Description of Related Art
In order to achieve a seismic sensor that is relatively small in size, yet capable of detecting infinitesimal seismic activity, it is advantageous for the spring-mass system to (1) have a long period and (2) be compact. Linear springs require an impractical length to achieve a long period while offsetting the force of gravity on the mass. Non-linear springs can have both of the required qualities, and a curved leaf spring is generally a non-linear spring. A constant and challenging problem with maintaining the accuracy and the calibration of a seismic sensor is the temperature sensitivity of the spring used to suspend the mass.
Temperature sensitivity severely limits the sensors ability to accurately detect seismic signals. In order to have a stable and sensitive sensor, temperature sensitivity must be minimized. Several different methods have been used in order to negate the effect temperature has on a spring of a sensor. One method uses two springs, each composed of a different material. Each material has a temperature coefficient that is opposite to the temperature coefficient of the other spring. Thus, as the temperature increases, one of the materials expands and the other material contracts. Thus, the two different temperature coefficients cancel one another out and, as a result, the effect of temperature on the spring is also canceled out.
Manufacturing a leaf spring that is composed of two different materials is expensive. Further, finding two materials having equal and opposite temperature coefficients over a broad temperature range is complex. Therefore, although this method minimizes the temperature sensitivity problem over a restricted range, it is an expensive, complex, and incomplete solution.
From the preceding descriptions, it is apparent that the devices currently being used have significant disadvantages. Thus important aspects of the technology used in the field of invention remain amenable to useful refinement.
The present invention introduces such refinement. In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits.
In its preferred embodiment, the seismic sensor has a neutral axis, a base having a pivot point, and a vertical axis. Attached to the base at the pivot point is a carriage which has a mass, a boom, a center of gravity and a neutral position. The carriage being in the neutral position when the center of gravity is along the neutral axis. The spring is also attached to the carriage. The spring suspends the carriage in the neutral position and impels the carriage into the neutral position. The spring is geometrically symmetrical about the neutral axis when the carriage is in the neutral position. The seismic sensor also includes a first point and a second point for attaching the spring to the carriage. Lastly, the seismic sensor includes a detector which is attached to the base, for sensing displacement of the carriage.
This invention minimizes the temperature sensitivity problem because of the symmetry of the spring or springs. Any expansion or contraction of the spring material is compensated for in that the expansion or contraction will be mirrored by the portion of the spring on the other side of the neutral axis. Any change in one side of the spring will result in an equal and opposite change in the opposite side of the spring, thus canceling the major effects of temperature on the sensor.
The following are preferences that are not necessary to practice the invention, they are however, preferred. It is preferred that the seismic sensor also include a force actuator, which is attached to the base, for supplying a force which urges the carriage into the neutral position.
It is also preferred that the detector is a displacement transducer for transforming the displacement of the carriage into an electrical signal. It is further preferred that the first point is a support member and a brace and the second point is a clamp.
It is preferred that the spring includes two springs each spring having a first end and a second end. The first end of each spring is fixed to the carriage via the support member and two braces. The second end of each spring is fixed to the carriage via the clamp. It is preferred that the linear spring coefficient be negligible when the carriage is in the neutral position.
It is also preferred that the displacement transducer include a first and a second capacitor plate and the boom include a third capacitor plate. The first, second and third capacitor plates work in conjunction to transform the displacement of the carriage into an electrical signal. It is further preferred that the pivot point have negligible rotational friction.
Also preferred is that the neutral axis be approximately thirty-five degrees from the vertical axis, and that the ends of the two springs be offset from one another. Further preferred is that the spring be composed of a material having a thermoelastic coefficient of approximately zero, such as, Ni-Span-C alloy.
It is preferred that the force actuator receive output from the displacement transducer which alters the amount of force supplied by the force actuator. The force supplied by the force actuator is in relation to the output from the displacement transducer thereby the greater the displacement of the carriage the larger the force supplied by the force actuator. It is also preferred that the force supplied by the force actuator is a magnetic force which is created by a coil-magnet.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings.